1. Field of the Invention
The present invention relates to a DS-CDMA cellular system and, more particularly, to an initial synchronization method for a DS-CDMA (Direct Sequence-Code Division Multiple Access) inter-base station asynchronous cellular scheme.
2. Description of the Prior Art
A CDMA cellular scheme using a wideband DS-CDMA (W-CDMA) scheme is used as a radio access scheme in next-generation mobile communication scheme IMT-2000.
Generally, cellular systems require two kinds of cell searches, namely, an initial cell search by which an initial cell to which a mobile station (MS) is to be connected is caught, and a neighboring cell search by which neighboring cells are searched out during a handover period. Notably in DS-CDMA cellular systems, each of the cells uses the same frequency. For this reason, initial synchronization is required concurrently with the cell search to reduce within ½ chip period the timing error between the spreading code of the received signal and the spreading code replica generated by a receiver.
Such DS-CDMA cellular systems can be categorized into two schemes, namely, inter-base station synchronous systems in which temporal synchronization is performed strictly among all base stations, and inter-base station asynchronous systems in which temporal synchronization is not performed.
Inter-base station synchronous systems achieve inter-base station synchronization using other systems such as the GPS (Global Positioning System). Since each of the base stations uses the same long code with different delays being given to the respective base stations, it suffices to synchronize only the timing of the long code during the initial search. In addition, the neighboring cell search during a handover period can be performed at a higher speed since the mobile station is notified of the code delay information of the neighboring base stations from the base station to which the mobile station belongs.
On the other hand, in inter-base station asynchronous systems, each of the base stations uses a different spreading code in order to identify the base stations. The mobile station needs to identify the spreading codes in performing the initial cell search. When searching neighboring cells during a handover period, the number of spreading codes used in the neighboring base stations can be limited from the base station to which the mobile station belongs. In any case, however, in comparison with inter-base station synchronous system, the search time is longer. When a long code is used for the spreading code, it takes a very long time in the cell search. However, this inter-base station asynchronous system has an advantage in that other systems such as GPS are not required. This allows flexible system expansion from cellular systems to indoor and underground shopping centers.
A cell search scheme capable of solving these problems inherent in the inter-base station asynchronous systems and performing initial synchronization at a high speed is being proposed by Kenichi Higuchi, Mamoru Sawahashi, and Fumiyuki Adachi, in “High Speed Cell Search Method Using Long Code Mask in DS-CDMA Inter-Base Station Asynchronous System”, Shingakugihou RCS96-122 (1997-01).
FIG. 1 shows an example of the format of a spreading code in a downstream control channel in the proposed conventional cell search method. FIG. 2 is a flow chart showing the proposed conventional three-stage cell search method.
In the proposed conventional cell search method, a long spreading code in a downstream control channel is masked in predetermined cycles, and a symbol is spread by only a short code common to the respective base stations. The mobile station detects the reception timing of a long code by using this mask symbol to realize a high-speed cell search.
The proposed conventional cell search method will be described in detail below with reference to FIG. 2.
First Stage: Detection of Long Code Timing
First of all, the mobile station detects the correlation of a received signal and a common short code by using a matched filter, and a long code timing detecting section 71 detects the correlation peak value at a mask symbol position. The temporal position at which a maximum correlation output is detected after correlation values corresponding to an arbitrary number of frames are averaged is set as the mask symbol reception timing of a connection destination channel.
Second Stage: Identification of Long Code Group
In the second stage, a long code group detecting section 72 detects the correlation between the received signal at the mask symbol reception timing detected in the first stage and each long code group identification short code. Correlation values corresponding to an arbitrary number of frames are averaged, and a frame timing and a long code group including a long code for spreading the received signal are identified from a long code group identification short code number at which a maximum correlation value is obtained.
Third Stage: Identification of Long Code
In the third stage, a long code is identified. In the mobile station, a long code detecting section 73 sequentially detects the correlation values between the received signal and the long code candidates included in the long code group detected by using the frame timing detected in the second stage. By comparing the detected correlation detection values with each other, threshold determination sections 74 and 75 perform threshold determination. With this operation, a long code is identified. In this case, for each long code timing detection, correlation detection with each long code is performed an arbitrary number of times (which has been set to two) until a cell search is detected. This operation is performed to prevent a cell detection miss caused by a decrease in the reception level of a desired wave due to fading. As a threshold, a maximum correlation peak value in long code timing detection is used. If a correlation detection value exceeds the threshold, a correlation for the long code is detected again to check cell detection. After the check, a frame synchronization detecting section 76 performs data frame synchronization detection. If a detection error occurs, a search count section 77 performs the third stage again by an arbitrary number of times. If this operation is performed beyond a predetermined number of times, the flow of processing returns to the first stage.
As described above, according to the three-stage cell search method proposed in the above reference, a cell search can be executed at a high speed, but faster initial synchronization is required.
In the conventional cell search method, as the probability of erroneous detection increases due to an increase in interference wave power with an increase in the number of users accommodated, a three-stage cell search must be repeated many times, resulting in an increase in search time.
In addition, as the number of long codes to be identified increases, the search time is undesirably prolonged.